Fair Market Valuation
As many cryptoassets are traded on exchanges without sterling pairs, the value of any gain or loss must be converted into sterling to declare on the Self-Assessment tax return.
Where the transaction does not have sterling value (for example if bitcoin is exchanged for Ethereum) an appropriate exchange rate must be established, in order to convert the transaction to sterling.
Reasonable care should be taken to arrive at an appropriate valuation for the transaction using a consistent methodology. Details of the valuation methodology should be kept.